Support custom Drop implementation

[taiki-e]

use pin_project_lite::pin_project;

pin_project! {
    pub struct Struct<'a> {
        was_dropped: &'a mut bool,
        #[pin]
        field: u8,
    }
    impl PinnedDrop for Struct<'_> {
        fn drop(this: Pin<&mut Self>) { // <----- NOTE: this is not `self`
            **this.project().was_dropped = true;
        }
    }
}

fn main() {
    let mut was_dropped = false;
    drop(Struct { was_dropped: &mut was_dropped, field: 42 });
    assert!(was_dropped);
}

It's clear how to pass options(=arguments in #[pin_project]), and we don't have to think about how to pass options. So, this is easiest to implement compared to other options. For all other options, we have to start by deciding how to pass the options, which can be a hard task, whether simple or complex to implement.

[jplatte]

What's t he current state of this?

[taiki-e]

The implementation is basically done except for this fixme and lack of documentation. The latter is probably easy, as we can just copy the description of this PR and pin-project's docs.

https://github.com/taiki-e/pin-project-lite/blob/30f66ddd051092ec49756b87233e7dd18c9ae40a/src/lib.rs#L1204

[taiki-e]

bors r+

Well, documentation is not a blocker. Filed #61 to follow-up.

[bors[bot]]

Canceled.

[taiki-e]

bors r+

[bors[bot]]

Build succeeded:

• ci

[taiki-e]

Released in v0.2.7